America’s Most Weeded: Crabgrass Back »

Crabgrass may grow up in the cracks between pavers.


For an annual weed, crabgrass certainly gets a lot of attention, most all of it is pretty negative and well-deserved. Crabgrass (Digitaria) can turn what was a beautiful lawn into a patchy mess in the matter of just a few weeks. It grows from seed each year, once the soil warms to about 60 or higher for at least a week in the spring but grows most rapidly during the heat of summer. The seed will usually germinate most quickly in bare soil areas or places near sidewalks or other places where the soil will warm up more quickly. A thick dense lawn is usually not where crabgrass will be most prevalent because it does not compete well with taller plants or shade.

Weed Profile

Crabgrass has much wider leaf blades than typical lawn grasses. Crabgrass seedlings along a walkway.

There are two main species of crabgrass that we will see in our lawns and gardens. Large or hairy crabgrass (Digitaria sanguinalis) has fairly broad, flattened leaves that have fine hairs along each leaf near where it connects to the main stem and a prominent mid-rib. Smooth crabgrass (D. ischaemum) has a similar appearance but lacks the hairy leaves and mid-rib. Both have much coarser grass blades than typical lawn grasses and are also usually lighter green in color so the crabgrass plants are quite noticeable. Crabgrass usually has a low-growing habit, spreading out along the ground, often covering a somewhat circular area up to a foot wide but it can also grow up to 2’ tall in some situations. As the stems elongate, arch over and grow along the ground, they will often produce roots at the nodes along the stems. It can be quite difficult to pull out a mature crabgrass plant because of all of those extra roots that the plant produces. After the stems elongate they will begin flowering, usually starting about the beginning of August and continuing until a light freeze kills the plant. The flower head looks like a hand with the fingers pointing upward – this relates to the botanical name of the plant, Digitaria. Seed will ripen within a few weeks with each individual plant capable of producing up to 1,500 seeds!

Once crabgrass begins to tiller out it will often form more roots along the stems. Crabgrass can form a dense mat of foliage stems and flower heads.

Crabgrass or Quack Grass?

Many people will confuse crabgrass with quack grass so it is important to note some significant differences. Crabgrass is a warm season, annual grass while quack grass is a cool season, perennial grass. So, quack grass may be found actively growing quite early in the spring while crab grass will still not have germinated. Quack grass produces white rhizomes or underground stems that allow it to spread quite easily while crabgrass does not produce rhizomes, just the spreading, above ground stems from the center of the plant. Quack grass may grow 3’ tall or more, particularly when it is in bloom while crabgrass will usually get no more than about 8 to 10” tall if it is not mowed. The seed head of quack grass is an elongated raceme, not the finger-like seed head of crabgrass.

Quack grass produces white rhizomes that grow beneath the soil. The seedhead of crabgrass looks like fingers pointing upwards.

Management


Crabgrass will often creep into the edge of a flower bed.

Control of crabgrass in lawns is of primary concern for most people. Homeowners spend lots of money each year trying to prevent crabgrass from growing in their lawns, usually using a weed-and-feed type product that is primarily a fertilizer that also contains an herbicide to kill crabgrass seedlings as they are just beginning to germinate. There are several active ingredients that are effective in crabgrass prevention, but one should check the label to be sure the weed-and-feed product they are applying is in fact for crabgrass prevention and not for the control of broadleaf weeds like dandelions. Those products will generally have no effect in controlling crabgrass. Commercial lawn care companies may also be able to apply the crabgrass preventer product alone, not in combination with a fertilizer but those products are usually not available to home owners.

Crabgrass preventer products are usually quite effective if they are applied at the right time. People often refer to the time when lilacs or Forsythia are blooming as an indicator as to the proper time to apply the product to their lawn. If you apply it too early, it may lose its effectiveness before the end of the summer, allowing for a late flush of crabgrass to emerge and grow. But, if you wait too long in the spring to apply it, you might miss the first flush of seedling germination and still have a major crabgrass problem. Once the seedlings are up and growing these products will not kill the seedlings. One other word of caution – crabgrass preventers will also prevent other kinds of weed seeds from germinating, as well as preventing the grass seed that you might apply that spring or summer to fill in bare areas or to thicken and existing stand of grass. If you plan to do some seeding, do it late in the fall to avoid problems with spring applied crabgrass preventers or simply do not use them at all in those areas.

There are a few post-emergence crabgrass killing herbicides available. They usually contain the active ingredient quinclorac. They can be effective but they need to be applied shortly after the crabgrass seedlings have emerged. Once the plants begin to produce multiple stems or tillers, control is greatly reduced. Mowing is certainly an option to slow down a crabgrass infestation or at least to give you the opportunity to catch the grass clippings and crabgrass seed heads to reduce the amount of overwintering seed for next year. Another important aspect of crabgrass control is to maintain a healthy lawn that is properly fertilized, watered and mowed.

Crabgrass is not just a problem in lawns but it can also be a big problem in gardens too. The bare soil in a garden is a great place for the seed to germinate and begin to grow. Timely cultivation will easily kill the young seedlings but older plants may re-root if they are hoed out and left on moist soil. Crabgrass also will invade gardens from the edge, creeping in from a grassy area around the garden, with the low growing stems that root down into the soil. So, if you see crabgrass beginning to grow around the edge of your garden, work to control it early in the summer before it spreads and roots down.

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